1. Field of the Invention
This invention is related to a CS6 antigen for use in vaccines to protect from pathological effects of enterotoxigenic E. coli. 
2. Description of the Related Art
CS6 is a component of CFA/IV (colonization factor antigen IV), one of three CFAs commonly found on enterotoxigenic Escherichia coli (ETEC). A recent study showed CS6 on 31% of ETEC isolated from soldiers in the Middle East. Other CFAs and similar proteins found on the surface of ETEC function as adhesins to attach bacteria to intestinal epithelial cells. Attached bacteria can then deliver their toxin(s) to the target cells. It has never been proved that CS6 is an adhesin for human tissue (Knutton, S., M. M. McConnell, B. Rowe, and A. S. McNeish, “Adhesion and Ultrastructural Properties of Human Enterotoxigenic Escherichia coli Producing Colonization Factor Antigens III and IV”, Infect. Immun. 57:3364-3371 (1989)), but a study in rabbits indicated CS6 is a colonization factor.
The CS6 operon has much in common with fimbrial operons from E. coli, Salmonella, Yersinia, Klebsiella, Haemophilus, and Bordetella. All contain molecular chaperons and ushers and a number of structural subunits. This area contains two sequences homologous to insertion sequences, but no complete insertion sequences.
The low GC content (34%) and codon usage that is characteristic of E. coli genes that are expressed at low levels suggest the CS6 genes may have originated in another species. GC content of 35-45% is characteristic of Gram positive bacteria such as Staphylococcus, Streptococcus, Bacillus, and Lactobacillus. Low GC content is common for virulence-associated genes of E. coli. 
CS6 is unusual because it is expressed on bacteria grown on a variety of media, unlike other CFA's from ETEC that are only expressed on bacteria grown on CFA agar. This unusual regulation is not peculiar to strain E8775 because ETEC isolated in 1990 expressed CS6 when grown on L agar. Temperature regulation of CS6 expression is characteristic of other CFA's from ETEC and virulence genes in a variety of pathogenic bacteria.
Although CS6 has never been visualized by negative staining, electron microscopy using anti-CS6 sera and colloidal gold indicated that it is present on the surface of ETEC. The apparent major protein associated with CS6 is approximately 16 kDa which is in the range of molecular weights typical for subunits for fimbriae and fibrillae. CS6 from ETEC strain E10703 of serotype O167:H5 has been cloned (Willshaw, et al., FEMS Microbio. Let. 49: 473-478 (1988)). Only 3 kb of DNA was necessary for expression of CS6.
That is in contrast to fimbriae that typically require approximately 9 kb of DNA and include genes for subunits as well as proteins for transport of subunits and synthesis and assembly on the bacterial surface.
Grewal teaches bacterial strains transformed with plasmids containing genes which encode CS6. However, that reference does not teach use of plasmids under the controls of a lac promotor and a CS6 promotor.